We have demonstrated that during the second half of pregnancy in the mouse and rat, but not in the pseudopregnant rat, a major secretion of testosterone occurs. We will attempt to determine the source of the hormone (e.g., maternal ovaries or adrenals, placenta), the function of the hormone during this period, and the control of the onset of hormone secretion, and to the mid-pregnancy LH surge in the mouse). In unrelated experiments the effect of intracellular injection of melanocyte stumulating hormone on dispersion of pigment granules in frog melanophores will be determined. Thus, we aim to answer a major problem in studies of the mechanism of action of hormones: must a polypeptide hormone interact with a plasma membrane "recptor" in order to produce a biological effect on a target cell without entering the cell, or is a "recptor" simply one way of internalizing the hormone (e.g., by pinocytosis) so that it can act intracellularly.